1. Field of the Invention
The present invention relates to shear stress sensors and more particularly to a micro sized multi-axis semiconductor skin friction/wall shear stress sensor/devices.
2. Description of the Related Art
A great number of techniques for the measurement of wall shear stress exist within the prior art, ranging from inferring the skin friction from measuring the boundary layer profile or using some correlation or analogy to the direct measurement of the force on a surface. Although all of these techniques can be shown to accommodate some flow regimes, indirect methods have not been shown to be reliable for complex flows, high-speed flows, or flows associated with combustion and/or impinging shocks. Alternatively, direct measurements do not require any foreknowledge of the flow or its properties and can provide accurate results all the regimes mentioned above.
Direct measurements, refers to techniques that separate a small element, referred to as a floating head, from the wall and measures the tangential force that the flow imparts on it. Direct measurements are the most believable of all the techniques. The sensor is measuring the actual shear on the surface, without respect to the fluid, the state of the boundary layer, or Reynolds Number. Since the floating head is level with the wall, the measurement is non-intrusive to the flow. The forces are very small, sometimes requiring large floating heads and expensive instrumentation to obtain accurate results. A variety of such direct shear stress sensors are also known in the art. One type of direct shear stress sensor is a floating element sensor such as disclosed in U.S. Pat. No. 4,896,098 incorporated herein by reference. Other flow sensors include those disclosed in U.S. Pat. Nos. 6,966,231; 6,408,698; 5,199,209 the entire contents of which are each incorporated herein by reference. These floating sensors are exposed to the fluid flow and thus are not suitable for extreme or hostile conditions and often require complex optical or capacitive transduction techniques. Another floating element shear-stress sensor employed differential optical-shutter-based floating element sensors for turbulence measurements such as disclosed in U.S. Pat. No. 6,426,796 the entire disclosure of which is incorporated herein by reference. However, the performance of this sensor suffers from front-side electrical contacts that interfere with fluid flow past the sensor and/or from remote mounting of the incident light source.
The measurement of shear stress is of importance in a large number of situations involving fluid flow, including aerodynamic, hydrodynamics, turbo machinery, and polymer processing among several others. Quantifying shear stress is important in order to understand and control the flow and in particular to control and suppress the development of turbulence in it. The magnitude of the shear stress and needed resolution in these various situations can span many orders from milliPascals to kiloPascal or more. The frequency response and spatial resolution needed also varies considerably. Both direct and indirect principles for sensing this shear force and sensors employing these exist with the direct methods being favored, however no commercial devices are readily available. The demands of sensors for aerodynamic application are in particular very challenging to meet as the forces involved are small (few Pascal to few hundred Pascal) and the resolution needed is high (few milliPascal) with high spatial resolution (few hundred microns or less) and frequency response from steady state to tens of kilohertz. There is also the need for directional information and the ability to array large numbers of such sensors to collect this information over the area of interest. The present invention is a micro sensor that is ideally suited for such this application and can fabricated using MEMS (Micro Electro Mechanical Systems) fabrication techniques. It is however not limited this application and can find use in the above mentioned or in other situations by a suitable choice of design parameters.